The amount of costly cellulose fibers present in a paper substrate, in part, determines the density of the substrate. Therefore, large amounts of costly cellulose fibers present in a paper substrate produce a more dense substrate at high cost, while low amounts of cellulose fibers present in a paper substrate produce a less dense substrate at low cost. Reducing the density of a coated and/or uncoated paper product, board, and/or substrate, inevitably leads to reduced production costs thereof. This is true in all paper substrate production and uses thereof. This is especially true, for example, in paper substrates used in envelopes, folding carton, as well as other packaging, applications. Substrates used in such as envelope and packaging applications have specified thickness or caliper.
By reducing the density of the paper substrate at a target caliper, less cellulose fibers are thereby required to achieve the target caliper. In addition to a reduction in production costs, there is a production efficiency that is appreciated and realized when a paper substrate's density is reduced. This production efficiency is due, in part, to a reduction in drying requirements (e.g. time, labor, capital, etc) of the paper substrate during production.
Examples of reducing density of the base paper substrate include the use of:                1) multi-ply machines with bulky fibers, such as BCTMP and other mechanical fibers in the center plies of paperboard;        2) extended nip press sections for reducing densification during water removal; and        3) alternative calendaring technologies such as hot soft calendaring, hot steel calendaring, steam moisturization, shoe nip calendaring, etc.However, these potential solutions involve high capital and costs. Thus, they may be economically infeasible.        
Still further, even if the above-mentioned costly reduction in density methods are realized, thus producing a paper substrate having a target caliper, the substrate is only useful if such methodologies foster an acceptably smooth and compressible surface of the paper substrate. Presently, there are few potential low-cost solutions to reduce density of a paper substrate having an acceptable smoothness and compressibility so that said substrate has a significant reduction in print mottle and acceptable smoothness.
Low density coated and uncoated paper products, board, and/or substrates are highly desirable from an aesthetic and economic perspective. However, current methodologies produce substrates that have poor print and/or printability quality. Further, acceptable smoothness targets are difficult to attain using conventional methodologies.
One methodology is to address the above problems at lower cost through the use of expandable microspheres in paper substrates. These methodologies, in part, can be found in the following U.S. Pat. Nos. 6,846,529, 6,802,938, 5,856,389, and 5,342,649 and Published Patent Applications: 20040065424, 20040052989, and 20010038893, which are hereby incorporated, in their entirety, herein by reference.
However, such microspheres are found, when applied in the papermaking process, to have relatively low retention in the resultant paper substrate. As a result, the expandable microspheres are lost to the white water and the efficiency of the introduction of expandable microspheres into the resultant paper substrate is low, thereby providing another costly solution to the above-mentioned myriad of costly solutions.
Accordingly, there is still a need for a less costly and more efficient solution to reduce density, increase bulk, and retain the good performance characteristics such as smoothness and print mottle within a paper substrate.